A standard 12-lead electrocardiogram (ECG) is a representation of the heart's electrical activity recorded from sensing electrodes on the body surface. It is a standard tool for evaluating the cardiac function. Normal ECG tracing is comprised of different waves that represent the sequence of depolarization and repolarization of the atria and ventricles. For example, a P wave represents atrial depolarization, a QRS complex represents ventricular depolarization, and a T wave represents ventricular repolarization. From these ECG waves, a plurality of intervals can be calculated that reflect the cardiac conduction properties (e.g., P wave duration, PR interval, QRS duration) and the repolarization properties (e.g., QT interval), heart rate (e.g., PP or RR intervals), etc. Collectively, these ECG waves and durations contain important diagnostic information regarding the underlying cardiac condition of a patient.
However, many patients have intermittent spontaneous cardiac arrhythmias, for example sinus bradycardia, non-sustained ventricular tachycardia or paroxysmal atrial fibrillation events, which may not be recorded during their clinic visits. In order to capture these infrequent arrhythmia episodes, external ECG monitoring devices, such as Holters, are frequently prescribed to continuously monitor the patient's ECG. However, Holter recording has two inherent drawbacks. First, the memory capacity is limited, and most commercially available Holter machines can only record 24-hr or 48-hr surface ECG. Second, the use of skin electrodes is inconvenient and uncomfortable for the patient, and is a significant source of measurement noise due to loose contact, muscle movement, and environmental factors. Consequently, the diagnostic yield of a Holter ECG is very limited.
To overcome these shortcomings, implantable loop recorder monitors have been introduced. By implanting a small device with sensing electrodes underneath the skin, the subcutaneous ECG monitor can record subcutaneous ECG which resembles the surface ECG. The subcutaneous ECG monitor can be configured as an implantable loop recorder (ILR), so that it continuously records newly acquired subcutaneous ECG while discarding the old recordings. When experiencing symptoms, a patient can use a handheld device which communicates with the subcutaneous device to trigger a snapshot of the recordings. Alternatively, the implantable cardiac device can be programmed to automatically trigger a snapshot of the subcutaneous ECG upon detection of an arrhythmic episode. The recorded snapshots can then be transmitted over the wired or wireless network to the physician's office for clinical review. Because the loop recorder continuously refreshes its memory, it can be carried for long periods of time. Thus, it is ideal for capturing ECG traces of infrequent episodes such as syncope. Recently, subcutaneous ECG recording has also become a useful means to monitor the cardiac rhythm after ablation of atrial fibrillation, to determine the ablation efficacy and adjust therapeutic schemes.
Irrespective of the ECG recording apparatus (e.g., ECG machines, bedside ECG monitors, Holter ECG monitors, subcutaneous ECG devices, etc.), reliable beat detection is the prerequisite for further ECG processing and clinical diagnosis. Despite decades of research, ECG beat detection has remained as a technical challenge. On one hand, many factors can cause over-sensing (false detection) of cardiac beats, such as, for example, large T waves, wider QRS complexes, muscle noise, electromagnetic interference (EMI), and the like. On the other hand, under-sensing (missed detection) of cardiac beats are also common for ECG signals that have a small signal-to-noise ratio. Existing methods for real-time ECG beat detection are either computationally complex, and therefore not suitable for the implementation in an embedded system, or oversimplified, so that they, for example, rely solely on ECG metrics such as peak amplitude, peak slope, etc., with or without adaptive sensing threshold, and thus they result in unsatisfactory performance.
Therefore, there is a need to provide an apparatus and a method for more accurate and efficient detection of cardiac beats based on surface ECG or subcutaneous ECG recordings.
In addition, there is also a need to optimize the geometric shape of subcutaneous ECG monitors or cardiac devices in general. On one hand, subcutaneous ECG monitors preferably should have a small size to facilitate implantation. On the other hand, large inter-electrode distance is preferred to facilitate signal sensing. No existing subcutaneous ECG monitor known in the state of art meets both of these two seemingly contradictory requirements. For example, two existing products, namely, Reveal ILR manufactured by Medtronic and Confirm ILR manufactured by St Jude Medical, both have an elongated rectangular shape. Each device has two electrodes located at the outer surface of the device along the long axis. Evidently, the inter-electrode distance is limited by the length of the device. The Sleuth ILR manufactured by Transoma has the shape of a typical pacemaker. One electrode is located at the outer surface of the device can, and another electrode is located at the tip of a wire antenna which is connected to the device header. Although this design can increase the inter-electrode distance, it also increases the difficulty of the implantation of the device due to the need to straighten the flexible wire antenna and secure its position.
Furthermore, all these mentioned products have only one sensing vector. The shape and size of the devices make it difficult to add additional electrodes. From U.S. Pat. No. 6,699,200, a boomerang shaped implantable loop recorder design having three sensing channels is known. However, this design has limited options to arrange the sensing electrodes (two electrodes at the end of the wings and one electrode in the center). In addition, the bending of the device between two wings makes it difficult to insert the device into the pocket. Moreover, the shape is not ergonomic, and the device may be easy to move in the pocket, causing change of the sensing vectors. Similar limitations also apply to the triangular shaped ILR disclosed in the U.S. Publication No. 2010/0312131.
For at least the reasons given above, there is a need to optimize the shape of the subcutaneous ECG monitor so that it has a small size and is easy for implantation, allows placement of multiple sensing electrodes with large inter-electrode distance, provides flexible sensing vectors, has an ergonomic shape that can give an improved cosmetic appearance after implantation, and can be securely anchored in the pocket and is less prone to movement.
The present invention is directed toward overcoming one or more of the above-identified problems.